1. Field of the Invention
The present invention relates to a method for producing a zinc-chromium alloy-plated steel sheet having excellent corrosion resistance and excellent plate adhesion.
2. Description of the Related Art
Galvanized steel sheets are widely used as rust-preventive steel sheets for automobiles, household electric appliances, construction materials and the like. This is effective because since a pure zinc layer is less noble relative to iron of the steel sheet, the zinc layer has a sacrificial anticorrosion effect in that the zinc is first corroded in formation of plating defects such as pinholes or the like and portions where the matrix iron is exposed by processing and these portions are covered by corrosion products, thereby preventing rusting of the steel sheet. However, the zinc layer has a fault that because pure zinc is active, the corrosion thereof very rapidly develops in a corrosive environment such as a spray of salt water or the like. In addition, another possible cause of insufficient corrosion resistance is that since pure zinc easily produces conductive ZnO as a corrosion product, the protective effect deteriorates due to the presence of the corrosion product on the surface. An improved plating method using Zn-Ni, Zn-Fe or the like has been proposed in place of the pure galvanized steel sheet. In recent years, Zn-Cr alloy plating and a method of producing a Zn-Cr alloy-plated steel sheet have also been proposed.
Methods of producing an electroplated steel sheet using a plating bath containing chromium are disclosed in Japanese Patent Laid-Open Nos. 57-67188, 64-55398 and 1-309998.
The method disclosed in Japanese Patent Laid-Open No. 57-67188 uses an electroplating bath containing 70 to 370 g/1 sulfate ion, 45 to 60 g/1 nickel ion, 0.5 to 13 g/1 chromium ion and 10 to 80 g/1 boric acid, the bath being kept at a pH value of 1.4 to 2. The amount of chromium contained in the plating bath used in this method is 1.0 wt. % at most, and any anticorrosion effect of chromium can hardly be expected. The chromium content must be further increased for improving the corrosion resistance.
Japanese Patent Laid-Open No. 64-55398 discloses a method of producing a zinc-chromium-plated steel sheet with excellent surface quality and corrosion resistance, wherein plating is effected with a current density of at least 50 A/dm.sup.2 by using an acid plating bath containing zinc ions, trivalent chromium ions and 0.01 to 20 g/1 of polyoxyalkylene derivative. This method permits the Cr content in the plating to be increased to about 40 wt. %. However, the plated layer exhibits poor adhesion, and is thus easily peeled off from a steel sheet in both the adhesion tests below.
In the so-called reverse TO adhesion test a cellophane tape is applied to the plated layer surface, the plated steel sheet is bent at 180.degree. on the cellophane tape side and is returned to its initial form, the cellophane tape is separated, and the amount of plated layer which adheres to the cellophane tape is weighed for determining the amount of peeling of the plated layer.
In the usual cellophane tape peeling test a cellophane tape is applied to the plated layer and is then forcibly separated therefrom, and the amount of peeling of the plated layer is determined from the weight of the plated layer which adheres to the cellophane tape.
In addition, since segregation of Cr occurs within a region of a high current density of at least 60 A/dm.sup.2 and causes a stripe pattern in the plating, this method is not necessarily a satisfactory plating method.
Japanese Patent Laid-Open No. 1-309998 discloses a method of producing an electroplated steel sheet with excellent corrosion resistance and surface glossiness, wherein electroplating is performed by using an acid plating bath containing Cr ions and a cation polymer and having a ratio of Cr.sup.6+ ion/ Cr.sup.3+ ion of 0.1 or less. The specification also discloses that a quaternary amine polymer is used as the cationpolymer. Although this method is capable of producing a Zn-Cr alloy-plated steel sheet, the method has the problems that the concentration of the cationpolymer cannot easily be kept constant because the cationpolymer is easily entrapped in the plated layer, and that although the adhesion of the layer plated with a low current density (50 A/dm.sup.2) is good, the adhesion of the plated layer obtained by plating with a current density of more than this value abruptly decreases. Further, although both Japanese Patent Laid-Open Nos. 64-55398 and 1-309998 take the amount of Cr deposition into consideration, improvements not only in corrosion resistance but also in adhesion are important problems. However, both specifications fail to describe improvement of adhesion.